1. FIELD OF THE INVENTION
The present invention relates to an electronic device adaptive to mounting on a circuit board, or a printed wiring board (PWB) and, particularly, to an electronic device which facilitates the mounting on the circuit board and transportation thereof by changing the shape of outer leads of an electronic device which has many leads.
2. DESCRIPTION OF RELATED ART
As a package for a semiconductor element such as an IC chip, an LSI chip or the like, there has heretofore been known a TAB (Tape Automated Bonding) type package in which the semiconductor element is mounted on a thin tape film and is molded with a resin. The tape film used for the package is chiefly composed of a polyimide resin and has a thickness of as small as about 100 .mu.m. On the tape film is laminated a copper foil which is then subjected to etching to form a number of leads. The ends on one side of the leads are called inner leads and are connected to the LSI chip by the bumps or the like and at the ends on the other side thereof outer leads extend that connect to the circuit board.
The TAB type package can have pins in a number which is greater than that of a conventional plastic flat package, i.e., the TAB type package can have, for example, about 300 to 400 pins or more than about 1000 pins maintaining a pitch among the leads of as narrow as, for example, 50 .mu.m. Therefore, the TAB type package gives such an advantage that a circuit designer is allowed to design a circuit without worrying much about the limitation in the number of terminals of an IC element. Moreover, since the number of terminals can be increased per a single IC element, it is possible to decrease the number of IC packages required on the board contributing to reducing the size and weight of electronic devices.
However, employment of such an electronic device packaging of the IC chip, LSI chip or the like into the TAB type package, gives rise to the occurrence of defects as described below.
First, such an electronic device has very fine leads which, therefore, have limited mechanical strength. Therefore, the leads are subject to being deformed while the electronic devices are being transported or when the leads are being formed. As shown in FIG. 1(A), for instance, a defective connection takes place (see FIG. 1(B)) if the leads 1 are deformed and float. Therefore, the electronic devices must be handled with extreme care.
To transport the electronic devices, therefore, a method has been employed, in which each electronic device is contained in a special carrier or a supporting tray, however, the probability of deformation of the leads due to vibration during transportation of the electronic device still exists. Therefore, a carry-in method has usually been put into practice according to which an electronic device without the completely finished leads but with the lead shape, i.e., a half-finished electronic device is placed on the carrier tape and is shipped, and the electronic device is cut off from the tape at the moment when it is to be mounted on a circuit board and then the leads are formed. In this case, however, the manufacturer who mounts the device on the printed board must be equipped with an apparatus for forming the leads. In either case, therefore, carrying the electronic devices involves cumbersome operation.
Second, when the electronic device is to be mounted on the circuit board, even when the leads are not deformed, it is difficult to position a number of leads on the circuit board while maintaining high precision, since the leads are very fine. That is, when the electronic device is mounted using a conventional automatic mounting device, it is necessary to mount it on the circuit board under the condition in which the ends of the leads are maintained flat and uniform by holding the whole device so that the leads are not deformed.
Third, since the leads have limited mechanical strength, the electronic device as a whole cannot be supported by the leads after it is mounted on the circuit board. As shown in FIG. 2(A), therefore, the electronic device must be mounted by placing an adhesive agent 4 in a gap between the circuit board 2 and the electronic device 3. If no adhesive agent is placed in the above gap (FIG. 2(B)), the electronic device 3 moves so that stability is lost and leads are broken due to vibration during transporting. When semiconductor devices that was not adhered to the board were subject to a vibration test in compliance with Japanese Industrial Standards (JIS C 7022), the leads were broken in almost all of the devices.